It would have to be Splinter Cell Chaos Theory. I love sneaky, ninja-like games. It was also the first game I bought for the original Xbox back in the day. For me, the only game that comes close to it Deus Ex: Human Revolution.
TES 4 Oblivion for me. Got me into gaming again after several years without even a computer, got me into learning about tweaking and hardware to play it on higher settings. Wouldn't want to play it today I think but I got a good 3-400 hours out of it. Awesome gaming experience.
The Legend of Zelda: Ocarina of Time, no contest. From memory I can hum Lon Lon Ranch, The Storm song, Minuet of forest, Bolero of Fire, epona's song and the lost woods ^.^
Not my favorite game ever but steam tells me that I have a Dawn of War addiction. Fantus PM me and test ya mettle, Da Boyz arez looking fuz sum fresh blood.
First thoughts were: Pirates!, Wing Commander, & Civilization (the very first version). Though I'd have to say Civilization + some hex editor (from Norton if I recall correctly) + batch files (used together to edit saved game files) was the most fun.
Has to be System Shock. At least that's the one that has stayed with me for all those years. Sadly, it was overlooked back in the day because of Doom, which, although fun, is a mediocre game by comparison.
Team Fortress. Played it on my friends pc a long time ago, this is the game that got me into pc gaming, and into pcs in general. Had never experianced gaming like that at the time, totally changed my thinking of gaming
Ah yes! Crazy Horse - Rest In Peace. I had this on the Speccy and then the Amstrad CPC-464 - I can't say it was my favourite game ever, but it was better than Kevin Toms' Football Manager which was the best I had played up until then!
Gotta be KOTOR 2 for me, don't think I've played a single player game through more times than that one, doing something different everytime and I swear I still never managed to do everything.
Goldeneye. Brilliant single player campaign (never did beat the cheat time on facility) and multiplayer is awesome fun. Always played license to kill with pistols only.